This invention relates to relatively small circuit breakers such as wiring circuit breakers and earth leakage breakers, and more particularly to the structure of such a circuit breaker which electrically connects a movable contactor, which is operated by a drive unit, to a connecting conductor secured to a casing.
FIGS. 16 and 17 shows one example of a conventional three-pole type circuit breaker (wiring circuit breaker). More specifically, FIGS. 16 and 17 are sectional diagrams taken along its central pole, showing a closed state and a open state thereof, respectively.
In FIG. 16, reference numeral 1 designates a resin-molded casing; 2, a cover; 3, a stationary contactor which is integral with a power-source-side terminal 3a, the contactor 3 being secured to the casing 1 with screws (not shown); 4, a stationary contact provided on the stationary contactor 2; 5, a movable contactor swingably mounted through a shaft 7 on a resin-molded holder 6; 8, a movable contact provided on the movable contactor 5 in such a manner as to confront with the stationary contact 4; 9, an overcurrent tripping device comprising a bimetallic member 9a, a L-shaped stationary conductor 9b welded to the bimetallic member 9a, a stationary magnet 9c surrounding the bimetallic member 9a, and an armature 9d confronted with the stationary magnet 9c in such a manner that it is swingable; 10, a connecting conductor piled on the stationary conductor 9b and secured to the casing 1 with a screw 11; 12, a flexible conductor both ends of which are connected to the movable contactor 5 and the connecting conductor 10 by blazing; 13, a load-side terminal secured to the casing 1 with a screw 14; and 15, a flexible conductor which is connected between the bimetallic member 9a and the terminal 13.
Further in FIG. 16, reference numeral 16 designates a switching mechanism for swinging the movable contactor 5 together with the holder 6. The switching mechanism 16 is normally latched by a tripping mechanism 18 including a cross bar 17 extended over the poles. A handle lever 19 is rockably supported in such a manner that it is swingable right and left. More specifically, the handle lever 19 is coupled through a switching spring 20 to the switching mechanism 16, and has a handle 21 at the top. A contact spring 22 is connected between the holder 6 and the movable contactor 5 so as to urge the movable contactor 5 towards the stationary contactor 3.
The movable contactors of the right and left poles (not shown) are provided on the right and left of the movable contactor 5. These movable contactors are also rotatably mounted through shafts on holders similar to that 6 shown in FIG. 16. Those holders of the three poles are coupled to one another through a switching shaft, which is rotatably fitted in bearing grooves formed in inter-phase partition walls of the casing 1. Further in FIG. 16, reference numeral 23 designates an arc extinguishing chamber provided over the range of movement of the movable contact 8.
In the circuit breaker thus constructed, current is allowed to flow from the stationary contactor 3 through the stationary contact 4, the movable contact 8, the movable contactor 5, the flexible conductor 12, the connecting conductor 10, the stationary conductor 9b, the bimetallic member 9a and the flexible conductor 15 to the terminal 13. If, in this case, overcurrent about ten times as large as the rated current flows in the circuit breaker, then the bimetallic member 9a is curved to the left in FIG. 16, thus pushing the cross bar 17. As a result, the switching mechanism 16 is released from the tripping mechanism, so that the movable contactor 5 is swung together with the holder 6 by the elastic force of the switching spring 20, thus quickly leaving from the stationary contactor 3. In this operation, arcs are formed between the stationary contact 4 and the movable contact 8; however, they are drawn into the arc extinguishing chamber 13 by the electromagnetic force induced.
In the case where large current such as short-circuit current flows in the circuit breaker, the stationary magnet 9c will attract the armature 9d. Therefore, in this case, the cross bar 17 is struck before the bimetallic member 9b is curved, thus causing the movable contactor to disengage from the stationary contactor 3 instantaneously. When the circuit breaker is opened by turning the operating handle 21 to the right as shown in FIG. 17, with the switching mechanism 16 latched the movable contactor 5 is raised by the elastic force of the switching spring 20, thus leaving from the stationary contactor as shown.
In the conventional circuit breaker described above, the movable contactor 5 swung by the switching mechanism 16 is electrically connected through the flexible conductor 12 to the connecting conductor 10 secured to the casing 1. The flexible conductor 12 is, in general, formed by weaving a number of bundles of thin copper wires. However, the use of the flexible conductor in the circuit breaker suffers from the following difficulties:
(1) As the movable contactor 5 is swung, the flexible conductor 12 is also swung. To increase the movement of the flexible conductor 12 would increase the swing of the flexible conductor 12, so that the flexible conductor 12 might be broken by an accumulation of a metal fatigue.
(2) In order to prevent the breakage of the flexible wire 12, the latter should not be greatly deformed when swung. For this purpose, it is essential to provide a space large enough to accommodate it. However, in this case, the rated current is necessarily increased, and the flexible conductor 12 must be increased in diameter accordingly. As a result, the casing 1 must be increased in size; that is, it is difficult to miniaturize the circuit breaker.
(3) The movable contactor 5 is resisted by the flexible conductor 12 when swung for a switching operation. The resistance given by the flexible conductor depends on the condition of connection of the flexible conductor with the mating parts and on the frequency of switching of the circuit breaker, thus affecting the contact pressure of the contacts 4 and 8 and the speed of movement of the movable contactor 5.
In addition, the present invention further relates to slidable contacts for connecting electric conductors in various electric equipment, such as circuit breakers or the like. More particularly, the invention concerns an improved surface treatment of such slidable contacts.
In electric equipment such as- circuit breakers, a disconnecting contact, a load switch, a connector, or the like, having a mechanically movable electric conductive portion, slidable contacts are used between movable and fixed conductor portions.
In the region of relative conductor movement, contacting conductive surfaces are momentarily changed during relative sliding movement so that contact resistance becomes unstable and tends to be high during relative surface movement by comparison with that in a stationary state. As a result of the increased contact resistance, the contacting surface portions are heated by electrical energy. If the contacting surfaces are made of copper or a copper alloy, surface oxidation occurs, the contact resistance is made higher by oxidation which, in turn, promotes further oxidation until the conductive surfaces no longer function as such. Conventionally, therefore, in devices designed to handle large current flow, the sliding contact surfaces are plated with silver (Ag) to prevent or at least reduce the oxidation.
Ag-plating, however, is so soft that it is subject to galling, and is worn away even under no-load switching to expose the foundation conductor. Further, Ag is softened by electrical heat during current conduction, leading to increased galling, and possible separation of the plating layer. Moreover, under high current loads, the contacting surface portions can be fused by heat generation. Although the heat generation can be suppressed to a certain extent by increasing the contact force of the sliding contact surfaces, movement of the slidable contact surfaces becomes more difficult with increased contact force, thus requiring increased sizes of drive mechanisms and spring mechanisms for increasing the contact force. Further, as the contact force is increased, the frictional force between the sliding conductive surfaces increases and results in abrasion of the plating layer irrespective of current loads.
To cope with the foregoing phenomenon, conductive grease has been applied to an Ag-plating coating film of the sliding contacts. Although intended to prevent galling and to stabilize contact resistance, experiments conducted by the present inventors have demonstrated that the use of such grease increased contact resistance during sliding of the contact surfaces, and that when a large current load is incurred, the Ag-plating film coated with grease tended to fuse more than the same contacts not coated with grease. Further, the grease has a tendency to become hardened after use for a long time at a high temperature.